Metallocene catalyzed propylene polymers and copolymers are known to have a low melting temperature, relative to conventional propylene polymers. For this reason, metallocene catalyzed propylene polymers and copolymers have been described as useful as the outer layer heat sealable material of multi-layer films in U.S. Pat. No. 5,468,440 (column 6, lines 32 to 41). Other multi-layer films in which the advantage of an outer layer of metallocene catalyzed propylene polymers are described are found in U.S. Pat. Nos. 5,529,843 and 5,462,807. Although these patents describe multi-layer films in which a skin layer is made from a metallocene catalyzed propylene polymer, the patents are silent on the use of metallocene catalyzed propylene polymers in inner layers of a multi-layer film. U.S. Pat. No. 5,254,394 to Bothe et al., discloses a clear polyolefin film for packaging comprising isotactic polypropylene base layer and a top layer of syndiotactic polypropylene which has high sealed-seam strength and high clarity. The top layer can contain lubricant additives, such as waxes at levels of up to 2% wt. relative to each other.
Opaque polypropylene films are described in U.S. Pat. No. 4,377,616 which teaches an opaque biaxially oriented polymeric film structure of lustrous satin appearance comprising a thermoplastic core matrix having a strata of voids which are created by void-initiating solid particles which are incompatible with the matrix material.
Oriented polypropylene films of the above-disclosed types are used extensively in the packaging of a variety of foods. These are often aimed at applications where it is desired to have as low a water vapor transmission (WVTR) and oxygen transmission (TO.sub.2) as possible. Other applications do exist for which these types of film structures are unacceptable. One of these is the wrapping of fresh cleaned and chopped produce, such a lettuce, carrots, celery, etc., from which salads can rapidly be prepared. For these applications, a higher level of WVTR and TO.sub.2 is desirable to allow respiration through the package walls, while continuing to maintain package integrity.
The film of the present invention has a WVTR and TO.sub.2 that have been increased by the use of novel means, as a way to address this need.